Environmental Impacts of Global Offshore Wind Energy Development until 2040

Continuous reduction in the levelized cost of energy is driving the rapid development of offshore wind energy (OWE). It is thus important to evaluate, from an environmental perspective, the implications of expanding OWE capacity on a global scale. Nevertheless, this assessment must take into account various scenarios for the growth of different OWE technologies in the near future. To evaluate the environmental impacts of future OWE development, this paper conducts a prospective life cycle assessment (LCA) including parameterized supply chains with high technology resolution. Results show that OWE-related environmental impacts, including climate change, marine ecotoxicity, marine eutrophication, and metal depletion, are reduced by ∼20% per MWh from 2020 to 2040 due to various developments including size expansion, lifetime extension, and technology innovation. At the global scale, 2.6–3.6 Gt CO2 equiv of greenhouse gas emissions are emitted cumulatively due to OWE deployment from 2020 to 2040. The manufacturing of primary raw materials, such as steel and fibers, is the dominant contributor to impacts. Overall, 6–9% of the cumulative OWE-related environmental impacts could be reduced by end-of-life (EoL) recycling and the substitution of raw materials.


I
Inflow SSP Socio-economic pathway

IEA
International energy agency

Methods and data 2.1 Estimation of OWE electricity production
The OWE electricity production was calculated based on three key parameters, i.e. capacity factor (CF), lifetime, and nominal capacity (NC). Multiple CFs ranging from 28-60% in the year range 2009-2020 were reported in the literature 1-9 depending on site characteristics (e.g. wind resources) and turbine technology (e.g. gearbox-based or direct drive nacelles). CF is expected to increase as larger wind turbine moving further from shore with better wind resources. Multiply component technology enhancement (e.g. use of permanent magnet-based and direct drive nacelles) will largely increase CF [10][11][12] . For simplicity, the medium value was used as the estimation of current (in 2020) CF and the maximum value was applied for the expected CF in 2040. This paper assumed dynamic CFs with 50% in 2020 that linearly increases to 60% in 2040. The designed lifetime of offshore wind turbines was estimated to be 20 to 25 years 13 . We applied dynamic lifetimes with a 20-year mean in 2020 that S3 increases to a 25-year mean in 2040, and a 5-year standard deviation Normal distribution, which is in line with our previous paper 14 . Ordinary least squares (OLS) regression was used to model future NC projections based on existed projects from 4C offshore 15 . More information on lifetime and NC modeling could be found in 2.2 and 2.4.1 in our previous research 14 , respectively. Figures S1 and S2 show the estimation of CF and nominal capacity, and lifetime, respectively.

Life cycle inventory analysis
Dynamic parameterized life cycle inventories were generated in this paper. Besides key parameters, i.e. CF, lifetime, and NC, turbine size (including rotor diameter and hub height) and distance from shore are another two main parameters. Several processes were adjusted by these two parameters (details provided in Para in Supporting Information II). OLS regression was used to model the future projections of turbine size based on existed projects from 4C offshore 15 (Figure 4 in 14 ). Future average distance from shore was estimated based on Fraunhofer IEE 16 ( Figure S3). Figure S3: Estimation of average distance from shore.

Manufacturing of turbines and foundations
The material requirements for manufacturing turbines and foundations from 2020 to 2040 were calculated based on the dynamic material flow analysis (dMFA) 14 and used in this paper. A wide assortment of materials was considered, which including bulk materials, rare earth elements (REEs), key metals, and other materials for manufacturing 24 component technologies in the nacelle, rotor, tower, and foundation.

Manufacturing of transmission
Material use for manufacturing offshore wind transmission (cables and substation infrastructures) was calculated based on the following assumptions and estimation. Internal cable material requirements were calculated based on cable length and material intensity. Internal cable length was determined by turbine layout. The spacing between turbines in a column would be 5 to 10 times of rotor diameters and spacing between columns would be 7 to 12 times of rotor diameters 17 . As turbine size grows, the spacing is likely to increase. Therefore, the upper bounds were used in this paper. Five types of internal cables (i.e. 3x95 mm2 Cu, 3x150 mm2 Cu, 3x240 mm2 Cu, 3x400 mm2 Cu, 3x630 mm2 Cu) were found currently in the market. In simplicity, the average material intensity value of these five types was used in this paper (shown in Table S1). External cables consist of submarine cables, onshore aerial and underground cables. Due to lacking of data and ignorable length, onshore aerial and underground cables were excluded in this paper. Submarine cable length was determined by distance from shore. We considered submarine cable length equal to distance S6 from shore. External cable material intensity (shown in Table S1) has been derived from 9 . A substation was assumed to consist of two ABB's transformers and one foundation per wind turbine. Data of the mass of materials and energy use were derived from ABB's report of Environmental Product Declaration 18 . Substation foundation was assumed to be identical to one fix-bottom based (monopile) foundation.

Installation
Installation of turbine and transmission is with less technological spectrum but the installation processes vary significantly among foundation types 2 .

Installation of foundation
The foundation installation processes vary among foundation types. Eight foundations 14 were classified into four types by their installation processes, i.e. foundation type I: Gravity-Base and High-Rise Pile Cap; type II: Monopile; type III: tripot and Jacket; Type IV: floating foundations (Semi-Submersible, Spar and TLP). For type I and II foundations, sour protection is needed before installation setup. Type II and III foundations need driving piles into the seabed. While type IV relies on mooring systems. Processes related to foundations installation were adapted from 19 . The details of foundation installation activities can be found in Table S2. This paper also included the impacts of land-use transformation during foundation installation. The land-use impact was characterized by land transformation and land occupation. The coverage of one foundation and its scour protection vary from roughly 1195 square meters (m2) (type I), 291 m2 (type II), 763 m2 (type III), to 22 m2 (type IV) 2 . The land transformation is measured for the land cover change from one type to another. The land occupation measures how long a certain amount of area has been covered by one land cover type. Details are shown in Table S3.

Installation of turbine
The turbine installation activities mainly include marine transportation of components from the harbor to erection site and component assembly by jack-up vessel. The installation time (work time) is nowadays only marginally more efficient per turbine as methods and procedures to install that were learnt and already well managed are not necessarily valid with the large turbines 20 . Therefore, this paper assumed turbine installation time is stable towards 2040. The fuel consumption of these processes was calculated based on 21 . The details of turbine installation activities can be found in Table S4.

Installation of transmission infrastructure
Installation of transmission includes installation of transformer, substations and cables. Installation of cables is related to laying the cabling and assembling contain processes 2 . Each process needs different equipment, which should be mobilized from where they are before beginning on-site operation and demobilized to where they are after finishing the work. Work time for these processes was collected from 22 (See Table S5).

Operations and maintenance (O&M)
O&M processes involve inspections and maintenance of the physical plant and systems are mostly dependent on turbine failure rates 23 . This paper considered preventative and corrective maintenance.

Preventative maintenance (scheduled)
Preventative maintenance (scheduled) consists of regular inspection of turbines, cables and substations. These processes were modeled based on vessel work time and fuel consumptions.

Corrective maintenance (unscheduled)
Corrective maintenance includes unscheduled inspection and repair of turbines, cables and substations (shown in Table S6). Generators and blades are two most vulnerable components of offshore wind turbines 24 . The biggest contributor to the failure cost for offshore wind turbines is the generator and gearbox (if any) major replacement in the nacelle 25 . Due to complex long-term working conditions, blades tend to experience many internal (e.g. the fatigue failure) and external (e.g. environmental conditions) damages 26 . Work time for replacement of blades was assumed the same as replacement of nacelle due to lacking of data. Replacement processes were modeled based on 2 .

Maintenance strategy
According to A Guide to UK Offshore Wind Operations and Maintenance 23 , workboats are the most economic option for near-shore sites while the support by helicopters (heli-support) is necessary for sites further from shore. Helicopter transport in cases where difficult weather conditions prevent access by workboats. Further, for offshore wind farms located in deep water, more helicopters are needed to support offshore wind farm O&M. Port-based workboats become the only practical option for cases. However, lacking detailed representations of different vessels involved makes assessments of these activities in LCAs tentative. 50% marine vessel and 50% Jack-up vessel were assumed to be used during replacement processes. 100 flight-hours per wind turbine along 25 years life time is reported in 9 . Thus, this paper assumed 4 flight-hours per year per turbine. S10

Decommissioning of turbines
Wind turbines should be entirely removed from the site and then dissembled onshore. A heavy lift vessel or dynamic positioning vessel will usually be used 28 . The procedure performed will depend on the size and weight of the turbine, and will determine the lifting capacity and vessel's deck space. The emissions of decommission processes are mainly related to transportation of decommissioned wind turbines. The details of turbine decommission marine activities can be found in Table S7.

Decommissioning of foundations
This paper assumed foundations will be decommissioned. However, deep foundations may be costly to remove and it may have severe impacts on marine environment. Normally, foundations can be kept in site and available for repowering (replacement of the existing turbines into more powerful ones). Foundation lifetime is longer than turbines with approximately 100 year 29 . When foundations reach EoLs, there are two removal options proposed: the complete removal and cutting from a certain depth below the mud line and leaving the rest in situ 28 . But these processes are out of the discussion of this paper. Details can be found in 28 .

Decommissioning of transmission pieces
This paper assumed cables will be left in situ. On the electrical side, array and export cables (transmission cables) could last more than 40 years, and the transformers 35 years 21 . Submarine cables (both internal and external cables) are usually buried into depths of more than a meter below the seabed, which will not pose safety risks for marine users and have limited environmental or pollution impacts 28 . The complete removal is considered to cause substantial damage and disruption to the seabed given the extensive length of the cables 30 .  kg SO2-Eq terrestrial ecotoxicity, TETPinf kg 1,4-DC. urban land occupation, ULOP square meter-year water depletion, WDP m3 water-. Climate change, marine ecotoxicity, marine eutrophication, and metal depletion were considered as the most relevant impact categories in this study. OWE is key to energy transition and considered as a promising renewable energy source to mitigate greenhouse gases emissions (GHGs). Climate change is a widely used impact category to represent GHGs. OWE is located over shallow open waters in the sea and moving further into deep waters. Marine ecotoxicity and marine eutrophication are two impact categories directly linked to marine environment. Metal depletion will likely be a concern as several metals (e.g. steel, copper, and aluminum) are required along with the large-scale expansion of OWE development. This study mainly focus on these four impact categories but the environmental impact results of other impact categories could be found in Results in Supporting Information II.  The environmental impact intensities are influenced by several combined effects: 1) increased lifetime leads to increased accumulative electricity production ( Figure S6). The environmental impacts per MW (Figure S7) only slightly increase from 2020 to 2030 for all impact categories, which verifies the significance of lifetime extension on impact intensity decline; 2) Nominal capacity will continue to increase in the future and leads to more powerful turbines, which have a larger rotor diameter corresponding to a high ratio of m2-of-swept-area-per-MW. According to Figure S4, climate change related GHG from 2030 to 2035, and from 2035 to 2040, is 2.0 (~13%) and 3.3 (~21%) kg CO2eq./MWh lower than the average value from 2020 to 2030, respectively, when nominal capacity increases from 7.8 MW in 2020 to 15.6 MW in 2040 (twofold to 2020). 3) Advanced and new technology development increase the capacity factor and further decrease the impact intensities ( Figure S5).     Figure S15: Sensitivity analysis on distance from shore. The proposed distance from shore was estimated by the OLS based on projects from 4C offshore 15 .

Limitations and outlook
Marine transportation is normally modeled through theoretical considerations of energy use in transporting a mass over an assumed distance (tkm) 34 . OWE case is specific as most of the time that marine vessels and supporting equipment spend is at site, e.g. unloading components on top of the foundation. Specialized vessels are often required during transportation, uploading, and maintenance. In ecoinvent, barge, transoceanic freight ship, and port facilities are only represented for marine transportation 34 . This study modeled marine transportation based on the work time of vessels and supporting equipment, and associated fuel consumptions (Table S2, S4, S5, and S6).
Currently, most offshore wind turbines are transported from harbor to site by tugboats 35 and installed by jack-up crane vessels in water depths up to 50m 36 . However, when wind turbines move further from shore with deep waters, specialized vessels are essential for transportation. Floating crane vessels are required to satisfy the high dynamic lifts of components for installing offshore wind S27 turbines in even deeper waters 37 . Moreover, larger turbine turbines in harsher environment will receive fatigue and corrosion damage, which require larger supporting infrastructures (e.g. specialized equipment and vessels) 32 . The availability of these infrastructures is a major challenge. There are numerous vessels in the small-scale market but more optimized vessels are still in the design phase. The scaling of background infrastructure and introduction of novel transportation technologies will likely further increase the impacts of installation, O&M, and decommissioning.